Photo Trash on Big Sur

And, if you haven't deleted any photos the Recently Deleted album will not appear.

If you're deleting from an album you need to use the key combination of ⌘+Delete. Otherwise the photos are only removed from the album.

Are you sure there are photos in it? In some versions of Photos, Recently Deleted does not appear at all if it is empty.

Regrettably I have forgotten whether that was the case back in Big Sur, and I don't currently have a test Mac running that system, but I think it was. On my Mac that is stuck back on Monterey (12.7), it is the case. Duplicating a photo and deleting the duplicate causes Recently Deleted to appear in the left sidebar at the bottom of the Photos section, just above the Albums section header.

You didn't say what system you were running before upgrading to Big Sur, but the location where deleted photos are held for 30 days has been called the Recently Deleted folder since well before Big Sur. Its icon is like a folder, not a trash can.

The total number of photos in the library isn't relevant, only the number deleted recently.

There's a documented situation in which deleted photos do not go to Recently Deleted. If you use iCloud Photos, and if you are out of iCloud storage space, then Photos will skip saving a copy of deleted photos in Recently Deleted. Could this apply to you?